Re: [PATCH mm-unstable] mm: folio_add_new_anon_rmap() careful __folio_set_swapbacked()

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/6/25 13:55, Barry Song wrote:
> On Tue, Jun 25, 2024 at 5:00 PM Hugh Dickins <hughd@google.com> wrote:
>>
>> Commit "mm: use folio_add_new_anon_rmap() if folio_test_anon(folio)==
>> false" has extended folio_add_new_anon_rmap() to use on non-exclusive
>> folios, already visible to others in swap cache and on LRU.
>>
>> That renders its non-atomic __folio_set_swapbacked() unsafe: it risks
>> overwriting concurrent atomic operations on folio->flags, losing bits
>> added or restoring bits cleared.  Since it's only used in this risky
>> way when folio_test_locked and !folio_test_anon, many such races are
>> excluded; but, for example, isolations by folio_test_clear_lru() are
>> vulnerable, and setting or clearing active.
>>
>> It could just use the atomic folio_set_swapbacked(); but this function
>> does try to avoid atomics where it can, so use a branch instead: just
>> avoid setting swapbacked when it is already set, that is good enough.
>> (Swapbacked is normally stable once set: lazyfree can undo it, but
>> only later, when found anon in a page table.)
>>
>> This fixes a lot of instability under compaction and swapping loads:
>> assorted "Bad page"s, VM_BUG_ON_FOLIO()s, apparently even page double
>> frees - though I've not worked out what races could lead to the latter.
>>
>> Signed-off-by: Hugh Dickins <hughd@google.com>
> 
> Thanks a lot, Hugh. Sorry for my mistake. I guess we should squash this into
> patch 1/3 "mm: use folio_add_new_anon_rmap() if folio_test_anon(folio) ==
> false"?
> Andrew, could you please help to squash this one?

Hope the commit message written by Hugh can also be squashed into the 
original patch, as it is very helpful to me :)

>> ---
>>   mm/rmap.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index df1a43295c85..5394c1178bf1 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1408,7 +1408,9 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
>>          VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
>>          VM_BUG_ON_VMA(address < vma->vm_start ||
>>                          address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
>> -       __folio_set_swapbacked(folio);
>> +
>> +       if (!folio_test_swapbacked(folio))
>> +               __folio_set_swapbacked(folio);
>>          __folio_set_anon(folio, vma, address, exclusive);
>>
>>          if (likely(!folio_test_large(folio))) {
>> --
>> 2.35.3
>>
> 
> Thanks
> Barry